Protecting circuit

ABSTRACT

A first resistor and a second resistor are connected in series across the collector and emitter of a transistor that is to be protected thereby to detect a voltage across the collector and emitter thereof, as well as to detect a collector current or an emitter current thereof, and a third resistor is connected to be collector or the emitter of the transistor that is to be protected. A detector transistor is driven by a voltage produced across the second and third transistors. A fourth resistor is connected between the collector and the base of the detector transistor, one end of the fourth resistor connected to the base of the detector transistor being connected to a constant-current circuit, and the other end of the fourth resistor connected to the collector of the detector transistor being connected to the base of a control transistor. The emitter of the control transistor is connected to the collector or the emitter of the transistor that is to be protected via the third resistor. The collector of the control transistor directly or indirectly controls the base current of the transistor that is to be protected.

BACKGROUND OF THE INVENTION

The present invention relates to a protecting circuit, and more specifically to a protecting circuit of the type of ASO (Area of Safety Operation).

Output power transistors used, for example, in a transistor power amplifier circuit must be operated within an ASO region so that permanent second breakdown is not developed in the collector/emitter voltage V_(CE) -collector current I_(c) characteristics. If the operation of the transistor falls out of the ASO region, the transistor develops second breakdown and is permanently destroyed.

FIG. 3 shows a protecting circuit of the type of ASO for restricting the operation of the transistor within the ASO region to protect the transistor from permanently destroyed, as has been disclosed, for example, in Japanese Utility Model Laid-Open No. 51-101752. The operating condition of an output transistor Q₄ is detected by a detector circuit made up of resistors R₁, R₂ and R₃. The detection output of the detector circuit is applied to the base of a control transistor Q₁ which controls a base current of the output transistor Q₄. In case the operation of the output transistor Q₄ is likely to fall outside the ASO region, the control transistor Q₁ is rendered conductive thereby to limit the base current of the output transistor Q₄. According to the above prior art, however, the forward voltage V_(BE) at the base-emitter junction of the control transistor Q₁ has been set to be equal to a threshold voltage at which the protecting circuit commences the protecting operation. Therefore, there arise such detects that the level of protecting operation is varied since the junction voltage across the base and emitter is dependent upon the temperature, or the level at which the protecting operation commences is greatly dispersed due to the dispersion in current amplification factor h_(FE) of the control transistor Q₂ or due to the dispersion in the base layer resistance r_(bb).

Japanese Patent Laid-Open No. 50-81247 discloses another protecting circuit of the ASO type as diagramatized in FIG. 4.

As has been disclosed in the above Laid-Open specification, a collector current I of an output transistor Q₄ of the circuit is found from the following equation (1), ##EQU1##

In this circuit, a voltage V_(BEQD) across the base and emitter of a transistor Q_(D) which equivalently operates as a diode when a circuit of collector and base is short-circuited, is smaller than a voltage V_(CE) across the collector and emitter of the output transistor Q₄. Therefore, the voltage V_(BEQD) is negligible. Thus, it is possible to reduce the effects upon the dispersion of element constants or upon the change in temperature (especially, it is possible to reduce the effects caused by the temperature dependency of the forward voltage at the base/emitter junction of the transistors).

According to the abovementioned prior art, however, since the power-limiting equation (1) assumes a curve of exponential function, it becomes very difficult to design the circuit which corresponds to a curve of allowable power loss of the collector of the output transistor Q₄ or which corresponds to the ASO region.

Through the research conducted by the inventors of the present invention, on the other hand, it become obvious that the power-limiting equation (1) which describes a curve of exponential function is disadvantageous because of the reasons mentioned below.

In a push-pull output amplifier circuit which operates on a power-supply voltage V_(cc) of positive polarity to drive a load resistance R_(L) via an output coupling capacitor, the two output transistors which are connected in series between the power-supply voltage V_(cc) and a ground potential GND are usually biased in a d-c manner such that the voltage across the collector and emitter of each of the transistors is about V_(cc) /2. Therefore, when an output voltage swing is raised and lowered in response to the input signals with the d-c operation level of V_(cc) /2 as a center, and when the input signals do not cause the voltage to rise to the power-supply voltage V_(cc) and do not, either, cause the voltage to lower to the ground potential GND, the output of the push-pull amplifier circuit acquires a non-clipped state. Under the non-clipped output condition, the locus of operation determined by the collector/emitter voltage V_(CE) -collector current I characteristics of a single output transistor does not exceed a straight line connecting the two operation points [O(V), V_(cc).2R_(L) (A)] and [V_(CC) (V), O(A)].

On the other hand, when excessive input signals are applied, the two output transistors are alternately rendered conductive and nonconductive responsive to the input signals. Therefore, the output of the push-pull output amplifier circuit is raised up to the power-supply voltage V_(cc) and is lowered to the ground potential GND responsive to the input signals, thereby to produce pulse waveforms clipped between the upper and lower levels. Under the clipped condition, the two transistors perform switching operations between the two levels, i.e., between the power-supply voltage V_(cc) and the ground potential GND. However, the locus of operation of a single output transistor under the clipped output condition caused by excessive input signals, does not exceed a straight line connecting the two operation points [O(V), V_(cc) /R_(L) (A)] and [V_(cc) (V), O(A)].

Particularly, with the audio output amplifier having small outputs (10 watts to 20 watts), it is desired not to operate the protecting circuit even under clipped output conditions caused by excessive inputs. Namely, when the protecting circuit is operated, a negative feedback loop is established to connect the output and the input of the output transistor, giving rise to the occurrence of oscillation phenomenon or causing the output to be extremely distorted.

In the audio output amplifiers, therefore, it is desired that the ASO characteristics lie between maximum allowable power loss characteristics (hyperbola) of the collector of the output transistor and the abovementioned operation locus (straight line) under the clipped output condition. Therefore, according to the prior art as shown in FIG. 4, the power-limiting equation (1) describes a curve of exponential function, causing the design of the circuit to be complicated.

Further, referring to the power-limiting equation (1), even when a resistor R₂ ' is constituted by an equivalent resistance of an aluminum wiring layer in a monolithic semiconductive integrated circuit to have a resistance of about 20 mΩ, the resistance of the resistor R₁ ' must be set to a considerably great value in order to obtain a limiting current of about 2 amperes. Hence, there remains such a problem that the dispersion in the resistance of the resistor R₁ greatly affects the limiting current.

In order to preclude the abovementioned problems, the present invention provides a protecting circuit of the ASO type.

SUMMARY OF THE INVENTION

The object of the present invention therefore is to provide a protecting circuit having linear protection initiating characteristics corresponding to a linear operation locus defined by the collector/emitter voltage V_(CE) -collector current I_(c) characteristics of a transistor, like output transistors of a push-pull output amplifier circuit.

In order to achieve the abovementioned object, the fundamental setup of the present invention consists of a detector circuit made up of a resistor for detecting the collector current or emitter current of the transistor that is to be protected, and voltage-dividing resistors for detecting the voltage across the collector and emitter, whereby the detection output is applied to a control transistor via an emitter-collector path of a transistor having a resistor across the collector and base thereof.

The invention is concretely mentioned below by way of Examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a protecting circuit according to an embodiment of the present invention;

FIG. 2 is a circuit diagram of a push-pull output amplifier circuit to which is applied the protecting circuit of FIG. 1;

FIG. 3 and 4 are diagrams showing conventional protecting circuits;

FIG. 5 shows protecting operation initiating characteristics of the protecting circuit of the present invention; and

FIG. 6 is a circuit diagram of a BTL circuit according to which a push-pull output amplifier circuit by the modified embodiment of FIG. 2 is constructed in the form of a monolithic semiconductive integrated circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a circuit diagram of an ASO-type protecting circuit according to an embodiment of the present invention.

In FIG. 1, symbol Q₄ represents an output transistor that is to be protected. A transistor Q₃ placed in a preceding stage with respect to the transistor Q₄ and is Darlington-connected therewith, works to drive the above-mentioned output transistor Q₄.

A constant-current circuit CS₂ connected to the base of the transistor Q₃ constitutes a load for a drive circuit (not shown) which drives the output transistors Q₃ and Q₄.

A resistor R₃ inserted between the emitter of the output transistor Q₄ and an output terminal 2 works to detect an emitter current which is nearly equal to a collector current of the output transistor Q₄. Further, resistors R₁ and R₂ connected in series between the collector and the emitter of the output transistor Q₄ work to detect a voltage across the collector and the emitter thereof.

A voltage at a connection point between the resistor R₁ and the resistor R₂ is applied to the emitter of a transistor Q₁. A resistor R₄ is inserted between the base and the collector of the transistor Q₁. A constant current I_(o) is allowed to flow through the resistor R₄ from the constant-current circuit CS₁.

The abovementioned resistors R₁, R₂, R₃, R₄ and the transistor Q₁, constitute a detector circuit for detecting the operating condition of the output transistor Q₄ that is to be protected.

The output of the detector circuit obtained at the collector of the transistor Q₁ is applied to the base of a control transistor Q₂. Consequently, the collector of the control transistor Q₂ works to control the base current of the output transistor Q₄.

In the thus constructed detector circuit, the level of the voltage applied to the emitter of the detector transistor Q₁ is increased by the emitter-base junction, decreased by the resistor R₄ and constant current I₀, and is applied to the base of the control transistor Q₂. The forward voltage between the base and emitter of the transistor Q₂ is as small as about 0.6 volt. Therefore, the resistors R₁ to R₃ are so selected as to satisfy the following relation (2)

    R.sub.1 >>R.sub.2 >>R.sub.3                                (2)

To satisfy the above relation (2), the resistances of resistors R₁, R₂ and R₃ have been selected to be 10.7 kΩ, 100Ω, and 15 m Ω(0.015Ω).

Hence, the voltage V_(BEQ2) appearing across the base and emitter of the transistor Q₂ can be approximately given by the following relation (3). ##EQU2## where Ic₄ represents an emitter current of the output transistor Q₄, V_(CEQ4) a voltage across the collector and emitter of the output transistor Q₄, and V_(BEQ1) a forward voltage across the base and emitter of the transistor Q₁.

When the voltage-current characteristics at the base-emitter junction of the transistors Q₁ and Q₂ are brought into agreement with each other, the control transistor Q₂ is rendered conductive to reduce the base current of the output transistor Q₄ so that the protecting circuit exhibits the protecting operation only when the forward voltages V_(BEQ1) and V_(BEQ2) across the base and emitter of transistors Q₁ and Q₂ satisfy the relation V_(BEQ2) >V_(BEQ1). Therefore, the current-limiting equation of the protecting circuit of the embodiment of FIG. 1, i.e., the condition by which the protecting circuit commences the protecting operation, can be given by the following relations (4) and (5), ##EQU3##

Here, if the constant current I_(o) of the constant-current circuit CS₁ is set to be I_(o) =Vcc/R₁, and the resistors R₂ and R₄ are so set as to be R₄ =2R₂ (for instance, R₂ =100Ω, R₄ =200Ω), the above relation (5) can be simplified as follows (here, Vcc represents a power-supply voltage applied to a power-supply terminal 1), ##EQU4##

In either of the relation (5) or (6), therefore, a straight line l₄ connecting two operation points P₁ and P₂ on the collector-emitter voltage V_(CEQ4) -collector current Ic₄ characteristics of the output transistor Q₄, represents a line for commencing the protecting operation as shown in FIG. 5.

For example, in the case of the relation (5), the two operation points P₁ and P₂ are located at ##EQU5## respectively. In the case of the relation (6), on the other hand, the two operation points P₁ and P₂ are located at ##EQU6## and [Vcc(V), O(A)], respectively.

Further, as shown in FIG. 5, the operation point P₁ is set to be smaller than a maximum allowable collector current Icmax of the output transistor Q₄ but is greater than Vcc/R_(L), and the operation point P₂ is set to a value Vcc which is smaller than a breakdown voltage BV_(CEQ4) across the collector and emitter of the output transistor Q₄.

As mentioned already, the operation locus l₁ of the output transistor Q₄ of the push-pull output amplifier circuit which operates on the positive power-supply voltage Vcc, does not exceed a straight line connecting the points [O(V), Vcc/R_(L) (A)] and [Vcc(V), O(A)] shown in FIG. 5, even during the clipped output condition caused by excessive input signals. The operation locus l₁ is so set as will usually exist in an ASO region l₃ determined by a maximum allowable collector power loss (hyperbola l₂) of the output transistor Q₄, maximum allowable collector current Icmax, and a breakdown voltage BV_(CEQ4) across the collector and emitter.

However, when the load R_(L) is short-circuited or the power-supply voltage V_(cc) is abnormally raised, the operation locus of the output transistor Q₄ falls outside the ASO region l₃, so that the output transistor Q₄ may be broken. Thus, just before the operation point of the output transistor Q₄ exceeds beyond the line l₄ connecting the two operation points P₁ and P₂, at which the protecting circuit starts to operate, the control transistor Q₂ is rendered conductive in accordance with the abovementioned relation (5) or (6), whereby the base current of the output transistor Q₄ is decreased to protect the output transistor Q₄.

As will be obvious from the current-limiting equation (5) or (6), since the forward voltage V_(BE) across the base and emitter of the transistor whose characteristics depend much upon the temperature is not included in the current-limiting equation, it is possible to materialize a protecting circuit having a protecting operation initiating level which has no relation to the changes in junction temperature of the transistor.

Further, in the above protecting circuit, dispersion in the absolute value of resistance R₃, dispersion in the ratio of resistance R₁ to resistance R₂, and dispersion in the ratio of resistance R₂ to resistance R₄, are the only causes of dispersion in the level of initiating the protecting operation. Consequently, the abovementioned protecting circuit enables the width of dispersion to be reduced as compared with the conventional circuits. In other words, when the resistance R₃ is formed by an equivalent resistance (for example, 10 to 30 mΩ) by means of an aluminum wiring layer in a monolithic semiconductive integrated circuit, the dispersion in resistance can be reduced. It is further possible to greatly reduce the dispersion in the ratio of semiconductive resistance R₁ to semiconductive resistance R₂ formed in the monolithic semiconductive integrated circuit. It is therefore desired to form the protecting circuit shown in FIG. 1 in a monolithic semiconductive integrated circuit, and particularly to form the resistors R₁ and R₂ by way of semiconductive resistors, and to form the resistor R₃ in the form of an equivalent resistance of a wiring layer of aluminum.

Further, since the current-limiting equations (5) and (6) of the protecting circuit exhibit linear characteristics as mentioned earlier, it is allowed to easily set the operation level of the protecting circuit responsive to the operation locus (straight line) required for the output circuit.

FIG. 2 is a circuit diagram when the protecting circuit of the embodiment of FIG. 1 is applied to a push-pull output amplifier circuit.

Symbols Q₄ and Q₆ represent output transistors. A d-c bias to the two output transistors has been so set that the voltage across the collector and emitter of each of them will be about Vcc/2 when no a-c signal is received. Transistors Q₃ and Q₅ work to amplify the input fed to a quasi-complementary push-pull output circuit.

The transistors Q₁, Q₂ and resistors R₁ to R₄ constitute a protecting circuit which was mentioned earlier in conjunction with FIG. 1, and a transistor Q₇ constitutes a constant-current circuit CS₁ which supplies the constant current Io to the resistor R₄.

A transistor Q₈ forms a constant-current circuit CS₂ which constitutes a load for drive transistors Q₁₁ and Q₁₂ which drive the push-pull output amplifier circuit.

A biasing circuit is constituted by diodes D₁ to D₃ which are inserted between bases of the input transistors Q₃ and Q₅ of the above output circuit.

The diodes D₄, D₅ and resistor R₇ constitute a constant-voltage circuit, and a transistor Q₁₀ which receives a forward constant voltage of diodes D₄, D₅ and an emitter resistor R₆ generate a constant current. The above-mentioned constant current Io is produced by a current mirror circuit in which the constant current is allowed to flow into a transistor Q₉, and the base and emitter of the constant-current transistors Q₇ and Q₉ are commonly connected.

A capacitor C, a resistor R₈ and a speaker SP constitute a load circuit.

The present invention should not be restricted to the abovementioned embodiment only; for instance, the current detecting resistor R₃ may be provided on the collector side of the output transistor Q₄.

In this case, a level shift transistor Q₁ and a control transistor Q₂ should be of the p-n-p type, and the constant-current circuits CS₁ and CS₂ should absorb constant current.

The protecting circuit of the present invention can be applied not only to the push-pull output circuit shown in FIG. 2, but also to various other types of circuits. In that case, the protecting circuit may be provided for both of the output transistors which perform complementary operation.

The input current to the output transistors Q₃ and Q₄ may be directly interrupted by the above transistor Q₂, or may be indirectly interrupted by means of a Schmidt trigger circuit or the like.

FIG. 6 shows an embodiment in which the protecting circuit of the present invention is applied to a BTL (Balanced Transformer-Less) circuit made up of the push-pull output amplifier circuit in the form of a monolithic semiconductive integrated circuit according to the modified embodiment of FIG. 2.

The circuit elements surrounded by a two-dot-dash line have all been formed in a single silicon chip by way of a customary method of making semiconductive integrated circuits.

An input signal applied to an input terminal 3" is transmitted to an input terminal 3"' of the integrated circuit. A differential phase separator circuit 100 made up of transistors Q₁₁ -Q₁₃ and resistors R₁₁ -R₁₄, works responsive to the signals and produces two output signals of anti-phase relation to two signal lines l₁₀ and l₁₁. A value of constant current flowing through the constant-current transistor Q₁₃ of the differential phase separator circuit 100 is determined by a biasing current which flows in a biasing circuit 101 made up of a transistor Q₁₄ and a resistor R₁₅.

A constant-voltage Zener diode ZD, a resistor R₁₆ and a transistor Q₁₅ constitute a constant-voltage regulator 102 which generates an operation voltage of nearly a constant value irrespective of the power-supply voltage Vcc fed to the power-supply terminal 1.

The operation voltage is fed to the differential phase separator circuit 100 and to the biasing circuit 101 via a resistor R₁₇.

Reference numerals 103 and 103' represent a pair of power amplifier circuits of the BTL circuit for directly driving the speaker SP without interposing any output coupling capacitors. The pair of power amplifier circuits 103, 103' have been constructed quite in the same manner. Therefore, only one power amplifier circuit 103 is closely diagrammatized, and the other circuit 103' is not diagrammatized.

The speaker SP is directly connected between a pair of outputs 2, 2' of the pair of power amplifier circuits 103 and 103'.

To prevent the oscillation, a series circuit consisting of a capacitor C₁₀₁ and a resistor R₁₀₁ is connected in parallel with the speaker SP, a series circuit consisting of a capacitor C₁₀₂ and a resistor R₁₀₂ is connected between an output terminal 2 and a ground terminal 4, and further a series circuit consisting of a capacitor C_(102') and a resistor R_(102') is connected between an output terminal 2' and the ground terminal 4.

Bootstrap capacitors C₁₀₃ and C_(103') have been connected between output terminals 2, 2' and bootstrap terminals 5, 5', respectively.

The two signals sent onto the two signal lines l₁₀ and l₁₁ are applied to the bases of transistors Q₁₆, Q_(16') in initial-stage amplifier circuits 104, 104' of the pair of power amplifier circuits 103 and 103'.

Although not specifically restricted thereto, the initial-stage amplifier circuit 104 includes a modified differential amplifier circuit consisting of transistors Q₁₆, Q₁₇ and a resistor R₁₈, a current mirror circuit consisting of transistors Q₁₈ and Q₁₉, and a load consisting of the resistor R₁₈ as shown. A nearly constant operation voltage is applied by a constant-voltage regulator 102 to the initial-stage amplifier circuit 104 via a resistor R₁₉. One end of the resistor R₁₉ is connected to a filter capacitor C₁₀₄ for removing power-supply rippling via a terminal 7, whereby the differential phase separator circuit 101 is allowed to very stably operate.

An output OUT produced from an output terminal 2 of the power amplifier circuit 103 is transmitted to the base of the transistor Q₁₇ of the inital-stage amplifier circuit 104 via a negative feedback circuit 105 consisting of resistors R₂₀ -R₂₂, a constant-current transistor Q₂₀, and a capacitor C₁₀₅ connected to a terminal 8, whereby an a-c voltage gain of the power amplifier circuit 103 is determined, and a d-c output voltage level at the output terminal 2 of the power amplifier circuit 103 is determined to acquire a value of nearly Vcc/2 as mentioned below.

That is to say, the base of the constant-current transistor Q₂₀ is biased by a biasing circuit 106 consisting of a transistor Q₂₁ and resistors R₂₃, R₂₄. Since the biasing circuit 106 has been connected to the power-supply terminal 1, a constant current which flows into the constant-current transistor Q₂₀ is dependent upon the power-supply voltage Vcc. Therefore, even when the power-supply voltage Vcc is varied, the d-c output level at the output terminal 2 follows the value Vcc/2 which varies responsive to the changes in the power-supply voltage Vcc.

On the other hand, the output signal of the initial-stage amplifier circuit 104 produced across the resistor R₁₈ is amplified by a drive amplifier circuit consisting of in Darlington-type connected transistors Q₁₁ and Q₁₂, a resistor R₅, a phase compensating capacitor C₁₀₆, a diode-connected transistor Q₂₂ and a constant-current load transistor Q₈. The output of the drive amplifier circuit 107 is fed to in Darlington-type connected transistors Q₃ and Q₄ in the push-pull output amplifier circuit 108, as well as to complementary-connected transistors Q₅ and Q₆.

The emitter of the transistor Q₅ is connected to an idling current adjusting circuit 109 made up of diodes D₆ -D₈, and transistors Q₂₃ and Q₂₄, such that the crossover distortion of the push-pull output amplifier circuit 108 can be reduced.

The base of the constant-current load transistor Q₈ of the drive amplifier circuit 107 and the base of the transistor Q₂₄ of the idling current adjusting circuit 109 have been connected to the base of the transistor Q₉ of the biasing circuit 110. The biasing circuit 110 is composed of transistors Q₁₀, Q₂₅, and resistors R₆, R₂₅, R₂₆, and is operated by a nearly constant operation voltage from a constant-voltage regulator 102. Therefore, a d-c biasing current flowing into the drive amplifier circuit 107 and a d-c biasing current flowing into a push-pull output amplifier circuit 108 are maintained at nearly constant values despite the changes in power-supply voltage Vcc.

The resistors R₁ to R₄ and the transistor Q₁ constitute a detector circuit 111 which operates on the same operation principle shown in FIGS. 1 and 2. The detector circuit 111, however, is different from the embodiments of FIGS. 1 and 2, in regard to that the transistors Q₁ and Q₂ are of the p-n-p type, the diode-connected transistors Q₂₆ and Q₂₇ are connected in series with the resistor R₁, the current detecting resistor R₃ formed in the form of an equivalent resistance in the aluminum wiring layer is connected to the collector of the transistor Q₄, and the constant-current circuit CS₁ for producing the constant current Io has been constructed in the form of a constant-current absorbing circuit.

Another difference is that the collector of the control transistor Q₂ which is driven by the output of the detector circuit 111 is not connected to the base of the transistor Q₃ but is connected to the base of the transistor Q₂₉ of the biasing circuit 112. Further, the collector of the control transistor Q₂ has been connected to the base of another transistor Q₂₈.

The biasing circuit 112 is further composed of transistors Q₃₀, Q₃₁, and a resistor R₂₉, and is operated by the operation voltage Vcc of the power supply.

The collector of the transistor Q₂₈ in the power amplifier circuit 103 and the collector of the transistor Q_(28') in the power amplifier circuit 103' are connected to a capacitor C₁₀₇ via the terminal 9. Since the transistors Q₂₈ and Q_(28') are usually rendered non-conductive, the capacitor C₁₀₇ has been electrically charged up to an operation voltage of the constant-voltage regulator 102 via a resistor R₃₀.

The terminal 9, on the other hand, is connected to an input of a Schmidt trigger circuit 113 via a resistor R₃₁. The Schmidt trigger circuit 113 is constructed, as shown, by means of transistors Q₃₂, Q₃₃ and resistors R₃₂ to R₃₅. The output of the Schmidt trigger circuit will be obtained at a connection point between the resistor R₃₄ and the resistor R₃₅. The resistance of the resistor R₃₂ has been set to be, for example, 10 KΩ which is greater than the resistance (for example, 2 KΩ) of the resistor R₃₃, such that the output of the trigger circuit acquires a low level when the capacitor C₁₀₇ has been electrically charged.

The detector circuit 111, on the other hand, has been so designed that a line l₄ for initiating the operation of the protecting circuit lies between the operation locus l₁ of the output transistor Q₄ and the line l₃ of the ASO region, like the case of FIG. 5.

Namely, the constant current Io which flows into the transistor Q₇ of the constant-current circuit CS₁ is given by the following relation.

    Io=Vcc-(V.sub.BEQ30 -VBEQ31)                               (7)

where

Vcc represents a power-supply voltage Vcc,

V_(BEQ30) and V_(BEQ31) represent forward voltages across the base and emitter of the transistors Q₃₀ and Q₃₁, and

R₂₉ a resistance of the resistor R₂₉.

When the resistors R₁ to R₃ satisfy a requirement R₁ >>R₂ >>R₃, the voltage V_(BEQ2) appearing across the base and emitter of the control transistor Q₂ is given by the following relation, like the equation (3) mentioned earlier,

    V.sub.BEQ2 ≈Ic.sub.4 ·R.sub.3 +(R.sub.2 /R.sub.1)(V.sub.CEQ4 -V.sub.BEQ26 -V.sub.BEQ27) +Io(R.sub.2 -R.sub.4)+V.sub.BEQ1                                      (8)

where V_(BEQ26) and V_(BEQ27) represent forward voltages across the base and emitter.

From the above equations (7) and (8),

    V.sub.BEQ2 ≈Ic.sub.4 ·R.sub.3 +(R.sub.2 /R.sub.1)(V.sub.CEQ4 -V.sub.BEQ26 -V.sub.BEQ27)+(R.sub.2 -R.sub.4)/R.sub.29 {Vcc-(V.sub.BEQ30+V.sub.BEQ31)}+V.sub.BEQ1 (9)

The resistances have been set as mentioned below, so that the resistors R₁ to R₃ satisfy the requirement R₁ >>R₂ >>R₃, the resistors R₂ and R₄ satisfy the requirement R₄ ≈2R₂, and the resistors R₁, R₂, R₄ and R₂₉ satisfy the requirement R₂ /R₁ ≈(R₂ -R₄)/R₂₉,

R₁ =10 KΩ (semiconductive resistor),

R₂ =100 Ω (semiconductive resistor),

R₃ =15 mΩ (equivalent resistance in the layer of aluminum wiring),

R₄ =200Ω (semiconductive resistor),

R₂₉ =10KΩ (semiconductive resistor).

Accordingly, if the base/emitter junction characteristics of the transistors Q₂₆ and Q₂₇ are equal to the base/emitter junction characteristics of the transistors Q₃₀ and Q₃₁, the current-limiting equation deriving from the equation (9) can be written as follows: ##EQU7##

As will be obvious from the current-limiting equation (10), the dispersion in base/emitter junction characteristics of the transistors Q₃₀ and Q₃₁ in the biasing circuit 112, and the temperature dependency, are cancelled by those of the transistors Q₂₆ and Q₂₇ in the detector circuit 111, whereby it is made possible to obtain a line for initiating the operation of the protecting circuit irrespective of the base/emitter junction characteristics of the transistors.

Quite in the same manner as the case of FIG. 5, just before the output of the output transistor of the power amplifier circuit 103 exceeds the line l₄ connecting the two operation points P₁ and P₂ for initiating the protecting operation, the transistor Q₂₈ is rendered conductive according to the equation (10), whereby the electric charge stored in the capacitor C₁₀₇ at the terminal 9 is discharged through the transistor Q₂₈. Hence, the output of the Schmidt trigger circuit 113 at a connection point between the resistor R₃₄ and the resistor R₃₅ acquires a high level to render the transistor Q₃₄ conductive.

The transistor Q₃₄ rendered conductive causes the transistors Q₂₅, Q₁₀ and Q₉ in the biasing circuit 110 to be nonconductive. Therefore, since the transistor Q₈ constituting the constant-current load CS₂ of the drive amplifier circuit 107 is rendered nonconductive, the transistors Q₃ and Q₄ of the push-pull output amplifier circuit 108 are rendered nonconductive.

Since the transistor Q₉ of the biasing circuit 110 has also been rendered nonconductive, the transistor Q₂₄ of the idling current adjusting circuit 109 is rendered nonconductive, and the transistors Q₅ and Q₆ of the push-pull output amplifier circuit 108 are rendered nonconductive, as well.

Thus, the base currents of the output transistors Q₄ and Q₆ of the push-pull output amplifier circuit 108 are interrupted, making it possible to prevent them from being destroyed. 

What is claimed is:
 1. In a protecting circuit comprising; means connected across the collector and emitter of a transistor that is to be protected thereby to detect a voltage across the collector and emitter of said transistor that is to be protected, current-detecting means connected in series with the collector-emitter current path of said transistor that is to be protected thereby to detect a collector current or an emitter current of said transistor that is to be protected, a detector transistor that will be driven with voltage produced by said two means, a constant-current circuit for supplying a constant current to said detector transistor, and a control transistor of which the base is driven by the collector output of said detector transistor thereby to directly or indirectly control the base of the transistor that is to be protected, the improvement characterized in that a first resistor and a second resistor are connected in series between the collector and emitter of said transistor that is to be protected thereby to constitute said means for detecting the voltage between said collector and emitter, an end of a third resistor that serves as said current detecting means is connected to the collector or the emitter of said transistor that is to be protected, other end of said third resistor is connected to the emitter of said control transistor, a connection point between said first resistor and said second resistor is connected to the emitter of said detector transistor, a fourth resistor is connected across the collector and base of said detector transistor, one end of said fourth resistor connected to the base of said detector transistor is connected to said constant-current circuit, and other end of said fourth resistor connected to the collector of said detector transistor is connected to the base of said control transistor.
 2. A protecting circuit as set forth in claim 1 wherein a resistance of the first resistor is sufficiently greater than that of the second resistor, and the resistance of said second resistor is sufficiently greater than that of the third resistor.
 3. A protecting circuit as set forth in claim 2 wherein the resistance of the third resistor is set by an equivalent resistance of a wiring layer in a semiconductive integrated circuit.
 4. A protecting circuit as set forth in claim 3 wherein the resistances of the first, second, third and fourth resistors, and a value of constant current of said constant-current circuit are so selected that a straight line for initiating the protecting operation lies between an operation locus of said transistor that is to be protected and an ASO region determined by the collector/emitter voltage-collector current characteristics of said transistor that is to be protected.
 5. A protecting circuit as set forth in claim 4 wherein the constant current of said constant-current circuit is determined by a biasing circuit including a resistor and a transistor which are connected in series between the power-supply voltage and a point of reference potential, and the base/emitter junction characteristics of said transistor of said biasing circuit are cancelled by the base/emitter junction characteristics of another transistor connected in series with said first resistor.
 6. A protecting circuit as set forth in claim 5, wherein a resistance of said resistor in said biasing circuit is selected to be equal to the resistance of the first resistor in said protecting circuit, and a resistance of said fourth resistor is selected to have a resistance which is about two folds of the resistance of said second resistor. 